Semiconductor devices are typically formed as multilayer structures in which the semiconductor materials are present in a patterned array that defines channels for the transport of charge carriers. For example, an inorganic semiconductor layer may be applied to a dielectric substrate surface. A mask may then be applied to protect patterned regions of the semiconductor layer, intended to constitute charge carrier channels, from a subsequently applied etchant. The etchant then removes the semiconductor in the unmasked regions, leaving behind a finely patterned array of semiconductor channels on the substrate. In the absence of such patterning, the semiconductor devices may be inoperable or be subject to excessive crosstalk.
Inorganic semiconductors typically are rigid and brittle at ambient temperatures. Hence, semiconductor devices formed with inorganic semiconductors generally are rigid as well. As the myriad end use applications for semiconductor devices have evolved, availability of semiconductor devices that can be flexed and bent without damage has become desirable. Flexible semiconductor device structures also offer a potential capability for fabrication of large area device arrays at low unit costs.
Much work has been done to develop organic semiconductors for applications where flexible semiconductor devices are needed. However, organic semiconductors generally cannot survive the harsh conditions required in order to carry out an etching step to generate a patterned array of channels for charge carrier transport. Printing processes have accordingly been sought in order to directly provide a patterned semiconductor channel array without a need to remove regions of a continuous layer of semiconductor material. However, in some cases the fine feature definition that is needed to generate microarrays of semiconductor channels has not been attained with acceptable reproducibility by direct printing of organic semiconductors.
Tetracene, for example, is an organic semiconductor of great interest due to its high charge carrier mobility in a single crystal. Single plate-like crystals of tetracene having dimensions as large as 5 millimeters long, 5 millimeter wide, and 0.25 millimeter thick have been produced. For example, individual thin film field effect transistors comprising single tetracene crystals have been made with high channel mobilities within a range of between about 0.1 centimeters squared per volt-second (cm2/Vs) and about 1 cm2/Vs, at room temperature. However, tetracene thin film field effect transistors in general have low mobilities and on/off current ratios, and their integration into a circuit may require further development. Furthermore, vacuum evaporation of tetracene leads to poorly defined and irregular boundaries between crystal grains that leads to low mobility and makes such grains unusable in fabricating semiconductor devices such as transistors or arrays comprising transistors.
One effort to generate a patterned array of semiconductor film channels comprising an aromatic acene semiconductor involved the direct printing of a pentacene precursor, which was then converted into pentacene. See, for example, Dimitrakopoulos et al. U.S. Pat. No. 5,981,970, issued on Nov. 9, 1999 and entitled, “Thin-film field-effect transistor with organic semiconductor requiring low operating voltages.” See also, Afzali, A., Dimitrakopoulos, C. D., and Breen, T. L., “High-performance, solution-processed organic thin film transistors from a novel pentacene precursor,” J. Am. Chem. Soc. 124 (30), pp. 8812–8813, Jul. 31, 2002. The entireties of both of the foregoing documents are hereby incorporated herein in their entirety.
Another process for forming patterned organic semiconductor films is disclosed in Katz U.S. Pat. No. 6,403,397 issued on Jun. 11, 2002, which is entitled “Process For Fabricating Organic Semiconductor Device Involving Selective Patterning”, the entirety of which is hereby incorporated herein. This process involves treating a surface to selectively provide regions of greater affinity and lesser affinity for either an organic semiconductor or an organic semiconductor solution. When the organic semiconductor, or solution comprising the semiconductor, was deposited on the treated surface, either the organic semiconductor or the organic semiconductor solution dewetted from the lesser affinity regions or the resultant film adhered only weakly to the lesser affinity regions such that selective removal was readily performed. Even where such removal was not performed, the portions of the organic semiconductor film on the greater affinity regions exhibited higher conductivity and better film continuity relative to the other portions of the film.
There remains a need for semiconductor devices comprising organic semiconductors having finely patterned regions of high charge carrier mobility. There further is a need for methods of making semiconductor devices with organic semiconductors that are not easily patterned.